This invention is concerned with pipes, tanks and the like fluid confining of the structures intended primarily for underground installation. When so installed, a fluid confining wall must have a degree of stiffness required to prevent buckling under the influence of soil loads and soil compaction. The load carrying capability of such structures must, for economic reasons, be realized with low cost structures utilizing minimum material costs, minimum fabrication cost, and minimum labor cost.
In the past, such structures have been designed in several different ways. But essentially, in use flexing loads are imposed upon the inner and outer surfaces of a pipe, and to increase the resistance to deflection, these surfaces must be separated. This is the principal of an I-beam, an H-beam, honeycomb panels, and the like. In conventional pipes, the wall thickness of the pipe is simply increased to achieve the requisite minimum stiffness. In glass fiber reinforced plastic pipe, it has been proposed that the wall thickness be increased by providing a composite wall wherein heavily glass fiber reinforced inner and outer surface layers of a pipe are separated by a medial layer of a filler, such as sand or the like. Such a structure is proposed in Carlstrom U.S. Pat. No. 3,406,724 and in Grosh U.S. Pat. No. 3,483,896. Another alternative proposal is the separation of inner and outer skins or shells by a foam coating applied to the inner skin, as in U.S. Pat. Nos. 3,295,558 and 3,598,275. Other proposals have included the use of cores or paper honeycomb cores. In all of these constructions, the problem has been the shearing of the core along the central or neutral axis of the pipe wall upon deflection loading of the pipe in its underground environment.
Another proposal has been the provision of a ribbed pipe or tank wall to achieve the required minimum stiffness. Here, the rib can fail due to excessive stresses at the rib crown. Thus, increasing the height of the rib to increase the stiffness causes stress failures at the rib crown. Further, the material required to achieve minimum stiffness and yet prevent failure due to excessive stress is substantial, and the cost efficiency of ribbed walls is doubtful.
It has also been proposed that inner and outer shells be interconnected by internal ribs inserted into a low strength core material. For example, Anderson U.S. Pat. No. 3,335,904 proposes trapezoidal ribs interposed in a foam or other low strength core material. Another proposal is the covering of an inner skin with a uniform layer of foam, grinding or cutting a slot into the foam material and then filling the slot with filament linings. These constructions have not been successful because of the high cost in producing them, and the difficulty of obtaining essentially radially extending integral ribs which are effective to form an integral monolithic structure with the shells. Such structures have not become commercial.